Stainless Steel Excellent in Corrosion Resistance, Ferritic Stainless Steel Excellent in Resistance to Crevice Corrosion and Formability, and Ferritic Stainless Stee Excellent in Resistance to Crevice Corrosion

ABSTRACT

The stainless steel of the first embodiment includes C: 0.001 to 0.02%, N: 0.001 to 0.02%, Si: 0.01 to 0.5%, Mn: 0.05 to 0.5%, P: 0.04% or less, S: 0.01% or less, Ni: more than 3% to 5%, Cr: 11 to 26%, and either one or both of Ti: 0.01 to 0.5% and Nb: 0.02 to 0.6%, and contains as the remainder, Fe and unavoidable impurities. The stainless steel of the second embodiment has an alloy composition different from those of the first and third embodiments and satisfies the formula (A): Cr+3Mo+6Ni≧23 and formula (B): Al/Nb≧10 and contains as the remainder, Fe and unavoidable impurities. The stainless steel of the third embodiment has an alloy composition different from those of the first and second embodiments and includes either one or both of Sn: 0.005 to 2% and Sb: 0.005 to 1% and contains as the remainder, Fe and unavoidable impurities.

TECHNICAL FIELD

The first embodiment of the present invention relates to a stainlesssteel that can be employed in salt-induced corrosion environments wheresuperior corrosion resistance is required. For example, the firstembodiment of the present invention relates to a stainless steel thatcan be employed in building materials or outside equipments used inmarine environments where there is ubiquitous airborne salt, or incomponents such as fuel tanks and fuel pipes of automobiles andtwo-wheeled vehicles which travel over cold regions where antifreezingagents are spread in winter.

The second embodiment of the present invention relates to a ferriticstainless steel that can be employed in components that demand superiorresistance to crevice corrosion and formability, such as equipments andpipings that have crevice portions in their design, for example,exhausts system components and fuel system components for automobilesand two-wheeled vehicles, hot water supply equipments, and the like.

The third embodiment of the present invention relates to a ferriticstainless steel that can be employed in components that demand superiorresistance to crevice corrosion, such as equipments and pipings thathave crevice portions in their design and are used in chlorideenvironments, for example, automobile components, water or hot watersupply equipments, building equipments, and the like.

This application claims priority from Japanese Patent Application No.2006-130172 filed on May 9, 2006, Japanese Patent Application No.2006-212115 filed on Aug. 3, 2006, Japanese Patent Application No.2006-215737 filed on Aug. 8, 2006, and Japanese Patent Application No.2007-26328 filed on Feb. 6, 2007, the contents of which are incorporatedherein by reference.

BACKGROUND ART

Stainless steel has been used in various applications in recent years,exploiting its excellent corrosion resistance. Local corrosions such aspitting corrosion, crevice corrosion, and stress corrosion cracking areparticularly important with regard to the corrosion resistance ofcomponents such as stainless steel devices or pipes, and there is aproblem that these give rise to penetration holes through which internalfluids can leak.

In marine environments, airborne salt which includes a large amount ofseawater components is the corrosive element. In cold regions, chloridescontained in antifreezing agents which are spread in winter are thecorrosive element. Sodium chloride and magnesium chloride are present aschlorides contained in seawater. These chlorides become adhered as anairborne salt component. When they then become wet, they readily formconcentrated chloride solutions. Meanwhile, antifreezing agents areformed of calcium chloride and sodium chloride, and since they aretypically applied in a solid state, they readily form a concentratedchloride solution. Among the chlorides varieties, sodium chloride driesat a relative humidity of 75% or less, while magnesium chloride andcalcium chloride will not dry until the relative humidity reaches 40% orless. As a result, magnesium chloride and calcium chloride formconcentrated chloride solutions over a wider humidity range. This alsoexpresses the extent of deliquescence, showing that magnesium chlorideand calcium chloride absorb moisture at a lower humidity to form aconcentrated chloride solution, compared with sodium chloride. Since therelative humidity is typically in the range of 40 to 75% in ambient air,it is extremely important to have a superior corrosion resistance in thepresence of concentrated magnesium chloride or concentrated calciumchloride.

Patent Document 1 discloses a ferritic stainless steel with improvedresistance to crevice corrosion. The invention disclosed in thisspecification is characterized in obtaining superior resistance tocrevice corrosion by adding a mixture of 16% or more of Cr and about 1%of Ni, without requiring a large addition of Cr or Mo. In this PatentDocument 1, evaluation was carried out using a repeated drying andwetting test in a sodium chloride environment. By employing a repeateddrying and wetting test, the corrosion characteristics of the disclosedferritic stainless steel in a concentrated sodium chloride solution canbe ascertained; however, no consideration is given to the corrosionproperties in a solution of concentrated magnesium chloride orconcentrated calcium chloride.

Patent Document 2 discloses a ferritic stainless steel which can be usedin marine environments due to the addition of a large amount of Cr andMo, and a suitable amount of Co. However, Co and Mo are expensive andmanufacturability is impaired with the addition of large amounts of Cr,Mo, and Co. Patent Document 3 discloses a ferritic stainless steel inwhich corrosion resistance is improved by the addition of P, andtherefore, large amounts of Cr and Mo are not required. Furthermore, byoptimizing amounts of C, Mn, Mo, Ni, Ti, Nb, Cu and N, manufacturabilitycan be assured. However, since P causes a deterioration in weldingproperties, this is a hindrance when manufacturing welded structures.Further, the most severe test of corrosion resistance that is disclosedin Patent Document 3 is the CASS test (sodium chloride solution spraytest), and no consideration is given to concentrated magnesium chlorideor concentrated calcium chloride environments. Patent Document 4discloses a ferritic stainless steel in which corrosion resistance isincreased by the addition of P, and the improvement of cleanness and thecontrol of configuration of inclusions are aimed to be attained byadding suitable amounts of Ca and Al. This Patent Document 4 alsodiscloses selective addition of Mo, Cu, Ni, Co and the like. Here, themost severe corrosion test is a crevice corrosion generating testconducted in 10% ferric chloride-3% sodium chloride solution, and noconsideration is given to concentrated magnesium chloride orconcentrated calcium chloride environments.

Austenitic stainless steel typified by SUS304 and SUS316L has excellentresistance to penetration hole formation caused by pitting corrosion orcrevice corrosion, but there is concern with respect to its resistanceto stress corrosion cracking. Accordingly, so-called “super” austeniticstainless steel which includes high-Cr, high-Ni, and high-Mo to suppressthe occurrences of the pitting corrosion and the crevice corrosion thatare the causes of the stress corrosion cracking may be considered to beemployed, or SUS315J1, 315J2 type steels in which stress corrosioncracking is improved by combined addition of Si and Cu may be consideredto be employed. However, both of these approaches are expensive.

Ferritic stainless steel has come to be used in various applications inrecent years due to its corrosion resistance, formability, and costperformance. Local corrosions such as pitting corrosion, crevicecorrosion, and stress corrosion cracking are particularly important withrespect to durability of stainless steel equipments and pipings. Forferritic stainless steels, pitting corrosion and crevice corrosion areparticularly important. In the case of components where crevice portionsare present in the design at welded sites, flange attachment sites, andthe like, crevice corrosion is particularly important, and there is aproblem that this crevice corrosion gives rise to penetration holesthrough which internal fluids may leak. For example, in the case ofautomobiles, there is a move to extend the guarantee period from 10 to15 years for essential parts such as fuel tanks, fuel supply lines, andthe like, and therefore, there is a need to ensure reliability over along period of time.

Further, local corrosions as described above are also important for thedurability of stainless steel equipments and piping components which areemployed in chloride environments.

In order to prevent penetration holes due to crevice corrosion, anddamage due to stress corrosion cracking arising from crevice corrosion,Patent Documents 5 and 6 disclose counter measures using coating andsacrificial corrosion protection.

In the case of coatings, there is a large burden on the environmentalmeasures since solvents and the like are used in the pre-treatmentprocess. Further, in the case of sacrificial corrosion protection, thereis a problem where maintenance costs are expensive. Therefore, it isdesirable to ensure resistance to crevice corrosion in an untreatedstate without relying on coating or sacrificial corrosion protection.Employment of a ferritic stainless steel in which corrosion resistanceis improved by adding large amounts of Cr and Mo may be considered asone approach. However, steels which include high-Cr and high-Mo have aproblem that formability is inferior and, moreover, are expensive.Therefore, a material which has both of corrosion resistance andformability without the addition of a large amount of an expensiveelement such as Mo has been desired.

Patent Document 7 discloses a ferritic stainless steel in whichcorrosion resistance is increased by the addition of P, and theimprovement of cleanness and the control of configuration of inclusionsare aimed to be attained by adding suitable amounts of Ca and Al. ThisPatent Document 7 further discloses the selective addition of Mo, Cu,Ni, Co and the like. However, the P causes a deterioration in weldingproperties, and is thus a hindrance when manufacturing weldedstructures. Further, costs rise due to the deterioration inmanufacturability. Further, while suitable amounts of Ca and Al may beadded to augment the decline in formability due to P, the suitable rangeis narrow, and production costs increase. Therefore, the ferriticstainless steel becomes expensive, and the merit of employing ferriticstainless steel is diminished due to its high cost as a material.

The above described Patent Document 1 discloses a ferritic stainlesssteel in which resistance to crevice corrosion is improved by theaddition of Ni, and discloses the selective addition of Mo and Cu forthe purpose of further improving resistance to crevice corrosion.Because Ni decreases formability, there is a problem that it becomesdifficult to form components where a high degree of formability isrequired, such as exhaust components or fuel system components ofautomobiles.

With regard to ferritic stainless steels containing Sn and Sb, aferritic stainless steel plate having excellent high temperaturestrength is disclosed in Patent Document 8, while a ferritic stainlesssteel having excellent surface properties and corrosion resistance, anda method for manufacturing the ferritic stainless steel are disclosed inPatent Documents 9 and 10. In Patent Document 8, improvement in hightemperature strength, and, in particular, a prevention of adeterioration in high temperature strength after long time aging israised as the effect of Sn. Similar attributes are ascribed to Sb. Theeffect in the present invention is an effect to the resistance tocrevice corrosion, and differs from the effects of Sn and Sb in PatentDocument 8. In contrast, Patent Documents 9 and 10 are characterized inemploying Mg and Ca as bases, adding Ti, C, N, P, S and O, and thencontrolling the contained amounts of these elements to improve ridgingcharacteristics and corrosion resistance. Sn is disclosed as aselectively added element. Improvement of corrosion resistance is raisedas the effect of Sn, and the corrosion resistance is evaluated usingpitting potentials in the examples. The pitting potentialelectrochemically evaluates resistance with respect to the generation ofpitting corrosion. In contrast, crevice corrosion is the subject ofstudy in the present invention. As will be explained below, one aspectof the present invention uncovers, as the efficacy of Sn, an effect oflimiting progression after the generation of crevice corrosion, and isdifferent from the effect of improving resistance to the generation ofpitting corrosion which is disclosed in

Patent Documents 9 and 10. Patent Document 1: Japanese PatentApplication, First Publication No. 2005-89828

Patent Document 2: Japanese Patent Application, First Publication No.S55-138058Patent Document 3: Japanese Patent Application, First Publication No.H6-172935Patent Document 4: Japanese Patent Application, First Publication No.H7-34205

Patent Document 5: Japanese Patent Application, First Publication No.2003-277992 Patent Document 6: Japanese Patent No. 3545759 PatentDocument 7: Japanese Patent No. 2880906 Patent Document 8: JapanesePatent Application, First Publication No. 2000-169943 Patent Document 9:Japanese Patent Application, First Publication No. 2001-288543 PatentDocument 10: Japanese Patent Application, First Publication No.2001-288544 DISCLOSURE OF THE INVENTION Problems to be Solved by theInvention

It is the first object of the present invention to provide a stainlesssteel having superior resistance to penetration hole formation arisingfrom crevice corrosion and pitting corrosion, as well as superiorresistance to stress corrosion cracking (stress corrosion crackingresistance) without adding a large amount of expensive Ni and Mo, insalt-induced corrosion environments such as a marine environment and aroad environment in cold regions where antifreezing agents are spread,in particular, even in such salt-induced corrosion environments astypified by highly concentrated magnesium chloride or highlyconcentrated calcium chloride, which are more severely corrosiveenvironments than that of the sodium chloride environment that was thetechnical subject of the prior art.

It is the second object of the present invention to provide a ferriticstainless steel having superior resistance to penetration hole formationat crevice portions (resistance to crevice corrosion) as well assuperior formability.

It is the third object of the present invention to provide a ferriticstainless steel having superior resistance to crevice corrosion, andparticularly superior resistance to penetration hole formation atcrevice portions.

Means to Resolve the Problem

The stainless steel excellent in corrosion resistance according to thefirst embodiment of the present invention includes, in terms of mass %,C: 0.001 to 0.02%, N: 0.001 to 0.02%, Si: 0.01 to 0.5%, Mn: 0.05 to0.5%, P: 0.04% or less, S: 0.01% or less, Ni: more than 3% to 5%, andCr: 11 to 26%, and further includes either one or both of Ti: 0.01 to0.5% and Nb: 0.02 to 0.6%, and contains as the remainder, Fe andunavoidable impurities.

Instead of a portion of the Fe, it may include one or more selected fromthe group consisting of Mo, Cu, V, W, and Zr, within the amounts of Mo:3.0% or less, Cu: 1.0% or less, V: 3.0% or less, W 5.0% or less, and Zr:0.5% or less.

It may further include one or more selected from the group consisting ofAl: 1% or less, Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% orless.

In the stainless steel that satisfies the above features, the combinedratio of austenite phase and martensite phase may be 15% or less,ferrite phase may be included as the remainder, and the grain sizenumber of the ferrite phase may be No. 4 or greater.

In the second embodiment of the present invention, resistance to crevicecorrosion is improved by the addition of Ni, and formability, which isnegatively impacted by the Ni, is secured by the addition of a suitableamount of Al and the optimization of the Al/Nb ratio. Thereby, aferritic stainless steel is provided that attains both of superiorformability and excellent resistance to penetration hole formation atcrevice portions (resistance to crevice corrosion).

The ferritic stainless steel excellent in resistance to crevicecorrosion and formability according to the second embodiment of thepresent invention includes, in terms of mass %, C: 0.001 to 0.02%, N:0.001 to 0.02%, Si: 0.01 to 1%, Mn: 0.05 to 1%, P: 0.04% or less, S:0.01% or less, Ni: 0.15 to 3%, Cr: 11 to 22%, Mo: 0.5 to 3%, Ti: 0.01 to0.5%, Nb: less than 0.08%, and Al: more than 0.1% to 1%, and contains asthe remainder, Fe and unavoidable impurities, wherein the amounts of Cr,Ni, Mo and Al satisfy the following Formulas (A) and (B).

Cr+3Mo+6Ni≧23  (A)

Al/Nb≧10  (B)

It may further include either one or both of Cu: 0.1 to 1.5% and V: 0.02to 3.0% at the amounts which satisfy the following formula (A′).

Cr+3Mo+6(Ni+Cu+V)≧23  (A′)

It may further include one or more selected from the group consisting ofCa: 0.0002 to 0.002%, Mg: 0.0002 to 0.002%, and B: 0.0002 to 0.005%.

In the third embodiment of the present invention, while considering thefact that by adding suitable amounts of Sn and Sb, resistance to crevicecorrosion is improved and the duration until formation of penetrationholes due to crevice corrosion is increased, a ferritic stainless steelexcellent in resistance to crevice corrosion is provided based on theeffect of the Sn and Sb on resistance to crevice corrosion,particularly, the effect on resistance to penetration hole formation atcrevice portions.

The ferritic stainless steel excellent in resistance to crevicecorrosion according to the third embodiment of the present inventionincludes, in terms of mass %, C: 0.001 to 0.02%, N: 0.001 to 0.02%, Si:0.01 to 0.5%, Mn: 0.05 to 1%, P: 0.04% or less, S: 0.01% or less, andCr: 12 to 25%, further includes either one or both of Ti and Nb withinthe amounts of Ti: 0.02 to 0.5% and Nb: 0.02 to 1%, further includeseither one or both of Sn and Sb within the amounts of Sn: 0.005 to 2%and Sb: 0.005 to 1%, and contains as the remainder, Fe and undetectableimpurities.

It may further include either one or both of Ni: 5% or less and Mo: 3%or less.

It may further include one or more selected from the group consisting ofCu: 1.5% or less, V: 3% or less, and W: 5% or less.

It may further include one or more selected from the group consisting ofAl: 1% or less, Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% orless.

EFFECTS OF THE INVENTION

The first embodiment of the present invention has excellent resistanceto penetration hole formation due to crevice corrosion and pittingcorrosion as well as excellent resistance to stress corrosion crackingin salt-induced corrosion environments. As a result, this embodiment iseffective in extending the lifespans of building materials and outsideequipments in a marine environment where airborne salt is ubiquitous, aswell as the lifespans of component parts such as fuel tanks, fuel pipes,and the like of automobiles and two-wheeled vehicles which travel overcold regions where antifreezing agents are spread in winter.

The second embodiment of the present invention can provide a ferriticstainless steel having both of excellent resistance to penetration holeformation at crevice portions (resistance to crevice corrosion) andsuperior formability. Thus, by employing the ferritic stainless steelhaving excellent resistance to crevice corrosion according to the secondembodiment of the present invention for components such as exhaustsystem components and fuel system components of automobiles andtwo-wheeled vehicles, hot-water supply equipments, and the like wherecrevice portions are present in the design and crevice corrosion isproblematic, their resistance to penetration hole formation can beimproved; therefore, the embodiment has the effect of extending thelifespan of the components.

In particular, the ferritic stainless steel according to the embodimentis suitable as a material for important components such as fuel tanksand fuel supply pipes of automobiles where a long lifespan is required.Furthermore, since formability is excellent, this material is easilyworked into a component, and is also suitable as a material for amanufactured part that is a steel pipe.

The third embodiment of the present invention can provide a ferriticstainless steel having excellent resistance to crevice corrosion,particularly excellent resistance to penetration hole formation atcrevice portions. Thus, by employing the ferritic stainless steel havingexcellent resistance to crevice corrosion according to the thirdembodiment for components, among components used for automobilecomponents, water and hot water supply equipments and buildingequipments, which have crevice portions in the design, and are used inchloride environments, and for which excellent resistance to crevicecorrosion is required, their resistance to penetration hole formation atcrevice portions can be improved. Therefore, the embodiment has theeffect of extending the lifespan of the components. Here, examples ofthe automobile components include exhaust system components and fuelsystem components, such as exhaust pipes, mufflers, fuel tanks, tankfixing bands, feed oil pipes, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the shape of the test piece.

FIG. 2 shows the conditions for the repeated drying and wetting test inExample 1.

FIG. 3 shows the conditions for the repeated drying and wetting test inExample 2.

FIG. 4 shows the relationship between Formula (A) and the maximumcorrosion depth.

FIG. 5 shows the results of the evaluation of the formability andresistance to ridging.

FIG. 6 is a schematic diagram showing the effects of Sn and Sb.

FIG. 7 shows the conditions for the repeated drying and wetting test inExample 3.

FIG. 8 shows the results for the repeated drying and wetting test.

FIG. 9 shows the relationship between the critical passivation currentdensity and the maximum corrosion depth at the crevice portion in therepeated drying and wetting test.

EXPLANATION OF THE SYMBOLS

-   1: spot welded part

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

Corrosion progresses due to active dissolution at sites where localcorrosions such as crevice corrosion and pitting corrosion occur.Austenitic stainless steel has a slow rate of dissolution, andtherefore, a long time is required until a penetration hole forms due todissolution at a corroded site. However, from the perspective ofpassivation that stops the dissolution, austenitic stainless steel isinferior to ferritic stainless. As a result, in austenitic stainlesssteel, active dissolution continues at a slow rate and susceptibility tostress corrosion cracking increases. In contrast, in ferritic stainlesssteel, since the active dissolution rate is high at sites where crevicecorrosion or pitting corrosion occurs, the time until a penetration holeforms due to dissolution at a corroded site is short. On the other hand,susceptibility to stress corrosion cracking is low in ferritic stainlesssteel.

As discussed in the prior art, magnesium chloride and calcium chloridecan exist as an aqueous solution at a lower relative humidity and have ahigher saturation concentration as compared to sodium chloride. For thisreason, since they can exist as a higher concentration chloride solutionover a wider humidity range, they have a stronger corrosivity thansodium chloride. Thus, the active dissolution rate at the area wherecrevice corrosion or pitting corrosion occurs is increased, and stresscorrosion cracking is promoted.

Rigorous research using ferrite stainless steel as the base wasconducted for an alloying element that was effective at promotingpassivation in order to reduce the active dissolution rate at areaswhere crevice corrosion or pitting corrosion occurs, and to improvesusceptibility to stress corrosion cracking. As a result of theseefforts, it was understood that Ni is the most useful element forreducing the rate of dissolution in the active state without impairingthe passivation ability, and that it must be included in an amount inexcess of 3% in order to provide a dissolution rate on par withaustenitic stainless steel in a salt-induced corrosion environmenttypified by concentrated magnesium chloride or concentrated calciumchloride. Further, it was discovered that the martensite and austentitephases are generated as second phases when the Ni amount is increased,causing a deterioration in the passivation ability, and that when theratio of the second phase is high, the steel becomes highly strong andhas low ductility, and therefore, there is a marked deterioration informability. It was further discovered that when the Ni amount is up to5%, there is a decrease in the active dissolution rate, and thedeteriorations in the passivation ability and in formability are withinpermissible limits. As a result, the present invention was attained.

The first embodiment of the present invention was conceived based on theabove understandings. The chemical compositions prescribed in thisinvention will now be explained in further detail below.

C: Because it decreases intergranular corrosion resistance andformability, it is necessary to keep the amount of C at low level.However, if the amount is extremely reduced, refining costs rise. Thus,the amount of C is prescribed to be in the range of 0.001 to 0.02%, andthe amount of C is preferably in the range of 0.002 to 0.015%, and ismore preferably in the range of 0.002 to 0.01%.

N: N is a useful element with respect to resistance to pitting corrosionand crevice corrosion. However, it lowers formability and intergranularcorrosion resistance. If the amount is extremely reduced, refining costsrise. Thus, the amount of N is prescribed to be in the range of 0.001 to0.02%, and the amount of N is preferably in the range of 0.002 to0.015%, and is more preferably in the range of 0.002 to 0.01%.

Si: Si is useful as a deoxidizing element, and is a useful element incorrosion resistance. However, since it reduces formability, its amountis limited to 0.01 to 0.5%. The amount is preferably in the range of0.03 to 0.3%.

Mn: Mn is useful as a deoxidizing element. However, when Mn is includedin excess, MnS is formed; thereby, it causes a deterioration incorrosion resistance. Therefore, its amount is limited to 0.05 to 0.5%.

P: Because it reduces welding properties and formability, it isnecessary to keep the amount of P at low level. Thus, the amount of P isprescribed to be in the range of 0.04% or less.

S: When S is present as readily soluble sulfides such as CaS and MnS, itserves as a starting point for pitting corrosion or crevice corrosion,thus causing deteriorations in resistance to pitting corrosion andresistance to crevice corrosion. Thus, the amount of S is prescribed tobe in the range of 0.01% or less. The amount is preferably 0.002% orless.

Cr: Cr is a fundamental element for ensuring corrosive resistance whichis most important for a stainless steel, and also, Cr stabilizes theferrite structure. Therefore, it is necessary to include Cr in an amountof at least 11% or more. While corrosion resistance improves as theamount of Cr is increased, formability and manufacturability decline.

Thus, the upper limit of the Cr amount is prescribed to be 26%. Theamount is preferably in the range of 16 to 25%.

Ni: In corrosive environments such as calcium chloride and magnesiumchloride that are more extremely corrosive than a sodium chlorideenvironment, Ni suppresses the active dissolution rate at sites wherecrevice corrosion or pitting corrosion occurs. In addition, Ni is themost effective element with respect to passivation. Therefore, Ni is themost important element in the present invention. In order to expressthese effects, it is necessary to include Ni in an amount of at leastmore than 3%. However, when Ni is included in excess, formabilitydeteriorates and costs rise. Accordingly, the upper limit of the Niamount is prescribed to be 5%. The amount is preferably in the range ofmore than 3% to 4% or less, and is more preferably in the range of morethan 3% to 3.5% or less.

Both of Ti and Nb fix C and N, and are useful elements from theperspective of improving formability and intergranular corrosionresistance at welded areas. The present invention includes either one orboth of Ti and Nb.

Ti: Ti fixes C and N, and is a useful element from the perspective ofimproving formability and intergranular corrosion resistance at weldedareas. It is necessary to include Ti in an amount of at least 0.01% ormore. It is preferable to include Ti in an amount that is four-fold orgreater than the sum of (C+N). However, when Ti is added in excess, Ticauses surface defects during manufacture, and leads to a deteriorationin manufacturability. Thus, the upper limit of the Ti amount is set tobe 0.5%. The amount is preferably in the range of 0.03 to 0.3%.

Nb: Nb fixes C and N, and is a useful element from the perspective ofimproving formability and intergranular corrosion resistance at weldedareas. It is necessary to include Nb in an amount of at least 0.02% ormore. It is preferable to include Nb in an amount which is eight-fold orgreater than the sum of (C+N). In the case in which both of Ti and Nbare included, it is preferable to include Ti and Nb in amountssatisfying the relation that (Ti+Nb)/(C+N) is six or more. However, whenNb is added in excess, formability declines. Accordingly, an upper limitof the Nb amount is prescribed to be 0.6%. The amount is preferably inthe range of 0.05 to 0.5%.

Mo: Mo may be included as necessary to ensure corrosion resistance. Byadding Mo in combination with Ni, it is possible to suppress the activedissolution rate at areas where crevice corrosion or pitting corrosionoccurs, and to increase the effect on passivation. Thus, corrosionresistance improves. Further, as in the case of Cr, Mo contributes tostabilization of the ferrite phase. Thus, if Mo is included, it ispreferable to include Mo in an amount of 0.5% or more. However, when Mois included in excess, Mo causes a deterioration in formability.Further, costs rise as Mo is expensive. Accordingly, if Mo is included,the amount is preferably in the range of 0.5 to 3.0%, and is morepreferably in the range of 0.5 to 2.5%.

V, W, Zr: V, W, and Zr may be included as necessary to ensure corrosionresistance. By adding any of these in combination with Ni, it ispossible to suppress the active dissolution rate at areas where crevicecorrosion or pitting corrosion occurs, and to increase the effect onpassivation. Thus, corrosion resistance improves. Further, V, W, and Zrcontribute to stabilization of the ferrite phase. Thus, if at least anyone of V, W, and Zr is included, it is preferable to add V in an amountof 0.02% or more, W in an amount of 0.5% or more, and Zr in an amount of0.02% or more. However, when included in excess, V, W and Zr cause adeterioration in formability and lead to rising costs. Thus, the upperlimits are set to be 3.0% for V, 5.0% for W, and 0.5% for Z.

Cu: Cu may be included as necessary to ensure corrosion resistance. Byadding Cu in combination with Ni, it is possible to suppress the activedissolution rate at areas where crevice corrosion or pitting corrosionoccurs, and to increase the effect on passivation. Thus, corrosionresistance improves. Thus, if Cu is included, it is preferable toinclude Cu in an amount of 0.1% or more. However, when Cu is included inexcess, formability deteriorates. Further, since Cu is an austeniteforming element, it is necessary to increase the amounts of Cr and Mo inorder to stabilize the ferrite structure. Thus, costs rise. Accordingly,if Cu is included, the amount is preferably in the range of 0.1 to 1.0%,and is more preferably in the range of 0.2 to 0.6%.

Al, Ca, Mg: Al, Ca and Mg have deoxidizing effects, and are usefulelements in refining. These may be included as needed. Further, Al, Caand Mg are also useful for refining the structure, and improvingformability and toughness. Therefore, it is preferable to include one ormore of Al, Ca and Mg within the amounts of Al: 1% or less, Ca: 0.002%or less, and Mg: 0.002% or less. Among these, Al is a ferrite generatingelement, and has the effect of suppressing the formation of austenitephase at high temperatures. As a result, the texture of ferrite phase isformed; thereby, this effect is thought to contribute to an improvementin formability. Here, if Al is included, the amount is preferably in therange of 0.002% or more to 0.5% or less. If Ca or Mg is included, eachamount is preferably in the range of 0.0002% or more.

B: B is an element useful for improving the secondary formability, andis preferably included in an amount of 0.0002% or more as needed.However, when included in excess, the primary formability deteriorates.Accordingly, the upper limit of the B amount may be prescribed to be0.005%.

The properties in which the combined ratio of austenite phase andmartensite phase is 15% or less, ferrite phase is included as theremainder, and the grain size number of the ferrite phase is No. 4 orgreater: As the amount of Ni increases, second phases such as theaustenite phase and the martensite phases become more readily present inaddition to the ferrite phase. In the case of the present invention,since Cr, Ni and Mo are not added in large amounts, the martensite phaseis more readily generated. When such a second phase is present,elongation at room temperature decreases, and therefore it is preferableto set the upper limit of the ratio of the second phases to be 15%.Further, if the temperature of the final annealing is increased in orderto suppress the generation of the second phases, the ferrite phasebecomes coarser, and the grain size number falls below No. 4. As aresult, the decrease in the elongation at room temperature becomesremarkable. Accordingly, the grain size number is preferably in therange of No. 4 or greater. The properties in which the ratio of thesecond phases is 15% or less and the grain size number of the ferritephase is No. 4 or greater are achieved by determining the Ni amountwithin the range of more than 3% to 5% that is prescribed in the presentinvention, to balance with the addition amounts of ferrite formingelements such as Cr and Mo and by setting the temperature of the finalannealing, or by, for example, the methods disclosed in the Examples.

Second Embodiment

In devices and pipes having crevice portions in their design, such asexhaust system components and fuel system components of automobiles andtwo-wheeled vehicles, hot water supply equipments, and the like, thepenetration hole formation (pitting) arising from crevice corrosion isan important factor determining the lifespan of the component. Thepresent inventors extensively researched the process of penetration holeformation due to crevice corrosion, while dividing this process into aninduction period up until crevice corrosion occurs, and a growth periodafter the occurrence of the crevice corrosion.

As a result, it became clear that in the case of ferritic stainlesssteel, the shortness of the latter period for corrosion growth is amajor cause of shortening the duration until the penetration holeformation. Thus, it was understood that suppressing the growth rate ofcrevice corrosion is an important factor for improving the duration ofresistance to penetration hole formation.

As a result of evaluating the impacts of various alloying elements, itwas discovered that Ni is most effective for suppressing the growth rateof the crevice corrosion, and that the resistance to crevice corrosionis improved by setting the value of Cr+3Mo+6Ni to be 23 or more.

Using a test piece formed by stacking a large test piece and a smalltest piece and spot-welding them at two points (the sites indicated by Oin FIG. 1), tests were carried out under the conditions shown in FIG. 3,and the maximum corrosion depth at the crevice portion was determined.The results are shown in FIG. 4. From these results, it can beunderstood that the maximum crevice corrosion depth is clearly reducedby setting the value of Cr+3Mo+6Ni to be 23 or more.

Next, various ferritic stainless steels were smelted, and the effect ofthe components on formability was investigated. As a result, it wasunderstood that formability was excellent when Al was added in anappropriate quantity. Further, it was understood that when the ratio ofAl and Nb satisfied a certain value, both of formability and resistanceto ridging were superior.

Various steels were prepared by using (16 to 19%) Cr−(0.8 to 2.8%)Ni−1.0% Mo−0.2% Ti steel as the base component, and adding variousamounts of Al and Nb. These steels were subjected to a process ofhot-rolling, annealing, cold-rolling, and annealing so as to form steelplates having the thickness of 0.8 mm. The results of evaluation offormability and resistance to ridging are shown in FIG. 5. Here,formability was judged as “good” or “bad” based on whether or notformation was possible in a cylindrical deep drawing test explainedbelow. Resistance to ridging was judged as “good” or “bad” based onwhether or not irregularities of 5 μm or more were present in thevertical wall portion after cylindrical deep drawing.

From the figures, it can be understood that good formability andresistance to ridging is obtained within the region surrounded by thethick solid line, that is, in the case where the Al amount is 0.1% to1.0% and the Al/Nb value is 10 or greater. It was thus understood forthe first time that there is an optimal range for the amount of Al fromthe perspective of formability and resistance to ridging, and thateither of these properties become poor when the amount of Al is eithertoo much or too little. Moreover, it also became clear for the firsttime that the ratio of Nb and Al, which heretofore has not been thefocus of much attention, is an extremely important index.

The mechanism by which formability is improved by the addition of asuitable amount of Al is not clear. However, it is thought that since Alis a ferrite forming element, it suppresses the formation of austenitephase at high temperatures; thereby, the texture of ferrite phase isformed which is beneficial to formability. It is also not clear whycontrolling Al/Nb leads to good formability and good resistance toridging, however, it is thought that differences of influences of Nb andAl on ability of solid solution strengthening, ability to generatecarbon nitrides, and rate of recrystallization contribute.

The second embodiment of the present invention was conceived based onthe above understandings. The chemical compositions prescribed in thisinvention will now be explained in further detail below.

C: Because it decreases intergranular corrosion resistance andformability, it is necessary to keep the amount of C at low level.However, if the amount is extremely reduced, refining costs rise. Thus,the amount of C is prescribed to be in the range of 0.001 to 0.02%.

N: N is a useful element with respect to resistance to pittingcorrosion. However, it lowers formability and intergranular corrosionresistance. Therefore, it is necessary to keep the amount of N at lowlevel. However, if the amount is extremely reduced, refining costs rise.Thus, the amount of N is prescribed to be in the range of 0.001 to0.02%.

Si: Si is useful as a deoxidizing element, and is a useful element incorrosion resistance. However, since it reduces formability, its amountis prescribed to be in the range of 0.01 to 1%. The amount is preferablyin the range of 0.03 to 0.3%.

Mn: Mn is useful as a deoxidizing element. However, when Mn is includedin excess, it causes a deterioration in corrosion resistance. Therefore,its amount is prescribed to be in the range of 0.05 to 1%. The amount ispreferably in the range of 0.05 to 0.5%.

P: Because it reduces welding properties and formability, it isnecessary to keep the amount of P at low level. However, if the amountof P is extremely reduced, raw material costs and refining costs rise.Thus, the amount of P is preferably in the range of 0.001 to 0.04%.

S: When S is present as readily soluble sulfides such as CaS and MnS, itserves as a starting point for pitting corrosion or crevice corrosion.Thus, the amount is prescribed to be in the range of 0.01% or less.

Cr: Cr is a fundamental element for ensuring resistance to crevicecorrosion, and it is necessary to include Cr in an amount of at least11% or more. Resistance to crevice corrosion improves as the amount ofCr is increased. However, with respect to resistance to penetration holeformation which is required in particular in the present invention, Crdoes not have a large effect on decreasing the rate of progression aftercrevice corrosion occurs. Further, since Cr deteriorates formability andmanufacturability, the upper limit of the Cr amount is prescribed to be22%. The amount is preferably in the range of 15 to 22%.

Ni: With regard to resistance to penetration hole formation at creviceportions (resistance to crevice corrosion), Ni is the most effectiveelement for decreasing the rate of progression after crevice corrosionoccurs. In order to express these effects, it is necessary to include Niin an amount of at least 0.15%. In particular, this effect is heightenedfurther when Ni is added in combination with Mo. The effect increases asthe amount of Ni is increased. However, when Ni is included in excess,susceptibility to stress corrosion cracking increases and formabilitydeclines. Further, this contributes to rising costs. Accordingly, theupper limit of the Ni amount is prescribed to be 3%. The amount ispreferably in the range of 0.4 to 3%.

Mo: Mo is particularly effective against the generation of crevicecorrosion. Also, by adding Mo in combination with Ni, the effect isenhanced which decreases the rate of progression after crevice corrosionoccurs. Thereby, it is possible to improve the resistance to penetrationhole formation at crevice portions (resistance to crevice corrosion).For this reason, it is necessary to include Mo in an amount of 0.5% ormore. However, when Mo is included in excess, formability deterioratesand costs rise because Mo is expensive. Accordingly, the amount of Mo isprescribed to be in the range of 0.5 to 3%. The amount is preferably inthe range of 0.5 to 2.5%.

Ti: Ti fixes C and N, and is a useful element from the perspective ofimproving formability and intergranular corrosion resistance at weldedareas. It is necessary to include Ti in an amount of at least 0.01% ormore. It is preferable to include Ti in an amount which is four-fold orgreater than the sum of (C+N). However, when Ti is added in excess, Ticauses surface defects during manufacture, and leads to a deteriorationin manufacturability. Thus, the upper limit of the Ti amount is set tobe 0.5%. The amount is preferably in the range of 0.03 to 0.3%.

Nb: Typically, Nb is often used, in the same manner as Ti, as an elementfor fixing C and N. In the present invention, when Nb is added inexcess, Nb causes a deterioration in formability and resistance toridging. Moreover, it is extremely important to prescribe the Al/Nbratio as will be described below, and adding a large amount of Nbinvites an increase in the added amount of Al. Thus, the upper limit ofthe Nb amount is prescribed to be 0.08%. Further, in order to carry outmanufacturing without a large increase in material costs, the Nb amountis preferably in the range of 0.01% or less. Here, Nb is often includedin the range of 0.001 to 0.005% as an unavoidable impurity in thetypical mass production manufacturing process.

Al: Al is known to have deoxidizing effects and to be a useful elementin refining, and there is a case where Al is included in an amount ofseveral tens of ppm. In the present invention, the formability of thecold-rolled steel plate is markedly improved when the added amount of Alis further increased, in particular, the effect was confirmed when theadded amount exceeds 0.1%. However, when Al is added in excess,formability conversely decreases, and toughness declines. Therefore, theamount of Al is prescribed to be in the range of 1% or less. The amountis preferably in the range of more than 0.1% to 0.5% or less. Themechanism by which formability is improved by the addition of Al is notclear. However, it is thought that since Al is a ferrite formingelement, it suppresses the formation of austenite phase at hightemperatures; thereby, the texture of ferrite phase is formed which isbeneficial to formability.

Al/Nb: The Al/Nb ratio is an index which was first elucidated by thepresent inventors. When this ratio is 10 or more, good formability andgood resistance to ridging can be obtained. Since this ratio becomesextremely large when Nb is not added, an upper limit is not particularlyprescribed. The reason is not clear why good formability and goodresistance to ridging are obtained by controlling the Al/Nb ratio,however, it is thought that differences of influences of Nb and Al onability of solid solution strengthening, ability to generate carbonnitrides, and rate of recrystallization contribute.

Cu: Cu may be included as necessary to ensure corrosion resistance. Byadding Cu in combination with Ni, the effect of decreasing the rate ofprogression after crevice corrosion occurs is enhanced; thereby, theresistance to penetration hole formation at crevice portions (resistanceto crevice corrosion) can be improved. For this reason, if Cu isincluded, it is preferable to include Cu in an amount of 0.1% or more.However, when Cu is included in excess, formability deteriorates andcosts rise because Cu is expensive. Accordingly, if Cu is included, theamount is preferably in the range of 0.1 to 1.5%.

V: V may be included as necessary to ensure resistance to crevicecorrosion. Similar to Mo, V is particularly effective with respect tothe generation of crevice corrosion, however, when included in excess,costs rise. Therefore, V may be included in an amount in the range of0.02 to 3.0%.

Further, either one or both of Cu and V are preferably included at theamounts which satisfy the following formula (A′), in order to furtherimprove the resistance to crevice corrosion.

Cr+3Mo+6(Ni+Cu+V)≧23  (A′)

Ca: As in the case of Al, Ca has deoxidizing effects and is a usefulelement in refining. Ca is preferably included as necessary in an amountof 0.0002 to 0.002%.

Mg: As in the case of Al and Ca, Mg has deoxidizing effects and is auseful element in refining. It also refines the structure and iseffective in improving formability and toughness. Accordingly, Mg ispreferably included as necessary in an amount of 0.0002 to 0.002%.

B: B is an element useful for improving the secondary formability, andcan be included as necessary. However, when included in excess, theprimary formability deteriorates. Accordingly, the B amount may beprescribed to be in the range of 0.0002 to 0.005%.

Third Embodiment

In the case of devices or pipes having crevice portions in their design,such as automobile components, water and hot water supply equipments,building equipments, and the like that are employed in chlorideenvironments, the penetration hole formation (pitting) arising fromcrevice corrosion is an important factor determining the lifespan of thecomponent. The present inventors extensively researched the process ofpenetration hole formation due to crevice corrosion, while dividing thisprocess into an induction period up until crevice corrosion occurs, anda growth period after the occurrence of the crevice corrosion.

As a result, it became clear that in the case of ferritic stainlesssteel, the shortness of the latter period for corrosion growth is amajor cause of shortening the duration until the penetration holeformation. Thus, it was understood that suppressing the growth rate ofcrevice corrosion is an important factor for improving the duration ofresistance to penetration hole formation.

As a result of evaluating the impacts of various alloying elements, thepresent inventors discovered that, like the case of Ni which isdisclosed in Japanese Patent Application, First Publication No.2006-257544, Sn and Sb are effective for suppressing the growth rate ofthe crevice corrosion, and that this effect is enhanced by thecombination with Ni or Mo, thereby improving resistance to penetrationhole formation at crevice portions. As is shown in schematic diagram ofFIG. 6, the growth rate of corrosion depth during the corrosion growthperiod which follows the induction period that is up until crevicecorrosion occurs is markedly reduced when Sn, Sb and Ni are added.

Cold-rolled steel plates were prepared employing0.005C-0.1Si-0.1Mn-0.025P-0.001S-18Cr-0.15Ti-0.01N as the basecomponent, and adding any one or more of Sn, Sb, Mo, Ni, Nb and Cu. Withthe exception of Mo, the amount of each element added was 0.4%. The spotwelded test pieces shown in FIG. 1 were employed using the cold-rolledsteel plates as materials, and a repeated drying and wetting test underthe conditions shown in FIG. 7 was carried out. The maximum corrosiondepth at the spot welded crevice was evaluated using the same method asin the Examples. These results are shown in FIG. 8.

Addition of Sn or Sb has the same effect on reducing the maximum depthof corrosion as does the addition of Ni, and this effect is furtherenhanced by adding both of Sn and Sb in combination. Further, a similareffect to that of Ni is obtained even when Sn or Sb is added incombination with Mo. Thus, it is understood that Sn and Sb are effectivefor improving the resistance to penetration hole formation at creviceportions, and this effect is further enhanced by the combination with Nior Mo.

Next, the relationship between the results of the repeated drying andwetting tests and growth behavior of crevice corrosion were investigatedelectrochemically. The material containing 1% of Mo was employed fromamong the materials employed in the repeated drying and wetting test,and an anodic polarization curve was measured in a 20% NaCl solutionhaving a pH of 1.5. This solution was designated as the simulatedinternal crevice solution after crevice corrosion occurs. Therelationship between the critical passivation current density (peakcurrent density in the active state) which is determined from the anodicpolarization curve, and the maximum corrosion depth at the creviceportion in the repeated drying and wetting test is shown in FIG. 9.

A strong correlation was confirmed between these. From this result, itwas understood that, like the addition of Ni, the addition of Sn or Sbhas the effect of suppressing the growth rate of crevice corrosion.

The third embodiment of the present invention was conceived based onabove understandings. The chemical compositions prescribed in thisinvention will now be explained in further detail below.

C: Because it decreases intergranular corrosion resistance andformability, it is necessary to keep the amount of C at low level.However, if the amount is extremely reduced, refining costs rise. Thus,the amount of C is prescribed to be in the range of 0.001 to 0.02%.

N: N is a useful element with respect to resistance to pittingcorrosion. However, it lowers formability and intergranular corrosionresistance. Therefore, it is necessary to keep the amount of N at lowlevel. However, if the amount is extremely reduced, refining costs rise.Thus, the amount of N is prescribed to be in the range of 0.001 to0.02%.

Si: Si is useful as a deoxidizing element, and is a useful element incorrosion resistance. However, since it reduces formability, its amountis prescribed to be in the range of 0.01 to 0.5%. The amount ispreferably in the range of 0.05 to 0.4%.

Mn: Mn is useful as a deoxidizing element. However, when Mn is includedin excess, it causes a deterioration in corrosion resistance. Therefore,its amount is prescribed to be in the range of 0.05 to 1%. The amount ispreferably in the range of 0.05 to 0.5%.

P: Because it reduces welding properties and formability, it isnecessary to keep the amount of P at low level. However, if the amountof P is extremely reduced, raw material costs and refining costs rise.Thus, the amount of P is prescribed to be in the range of 0.04% or less.

S: When S is present as readily soluble sulfides such as CaS and MnS, itserves as a starting point for pitting corrosion or crevice corrosion.Thus, the amount is prescribed to be in the range of 0.01% or less.

Cr: Cr is a fundamental element for ensuring resistance to crevicecorrosion, and it is necessary to include Cr in an amount of at least12% or more. Resistance to crevice corrosion improves as the amount ofCr is increased. However, with respect to resistance to penetration holeformation which is required in particular in the present invention, Crdoes not have a large effect on decreasing the rate of progression aftercrevice corrosion occurs. Further, since Cr deteriorates formability andmanufacturability, the upper limit of the Cr amount is prescribed to be25%. The amount is preferably in the range of 15 to 22%.

Ti, Nb: Ti and Nb fix C and N, and are useful elements from theperspective of improving formability and intergranular corrosionresistance at welded areas. It is necessary to include either one orboth of Ti and Nb in each amount of at least 0.02% or more. When onlyone of Ti and Nb is included, it is preferable to include Ti in anamount which is four-fold or greater than the sum of (C+N), and toinclude Nb in an amount that is eight-fold or greater than the sum of(C+N). When both of Ti and Nb are included, it is preferable to includeTi and Nb in amounts satisfying the relation that (Ti+Nb)/(C+N) is sixor more. However, when Ti is added in excess, Ti causes surface defectsduring manufacture, and leads to a deterioration in manufacturability.Likewise, when Nb is added in excess, Nb causes a deterioration informability. Thus, the upper limit of the Ti amount is set to be 0.5%and the upper limit of the Nb amount is set to be 1%. The Ti amount ispreferably in the range of 0.03 to 0.3%, and the Nb amount is preferablyin the range of 0.05 to 0.6%.

Sn, Sb: With regard to resistance to crevice corrosion, particularly,resistance to penetration hole formation at crevice portions, Sn and Sbare extremely useful elements for decreasing the rate of progressionafter crevice corrosion occurs. This effect is particularly enhancedwhen Sn or Sb is included in combination with Ni or Mo. In order toexpress this effect, it is necessary to include Sn or Sb in each amountof at least 0.005%. While this effect is enhanced as the amount of Sn orSb is increased, when included in excess, Sn and Sb cause adeterioration in formability and hot workability. Thus, the amount of Snis prescribed to be in the range of 0.005 to 2%, and the amount of Sb isprescribed to be in the range of 0.005% to 1%. The amount of Sn ispreferably in the range of 0.01 to 1%, and the amount of Sb ispreferably in the range of 0.005 to 0.5%.

Ni: Ni may be included as necessary to improve resistance to crevicecorrosion. With regard to resistance to penetration hole formation atcrevice portions (resistance to crevice corrosion), Ni is extremelyuseful element for decreasing the rate of progression after crevicecorrosion occurs. Ni has effects similar to Sn and Sb, even when usedalone. When Ni is added in combination with Sn and Sb, its effects areeven further enhanced. This effect becomes stable at the amount of 0.2%or more. The effect of Ni is enhanced as the amount of Ni is increased,however, when included in excess, susceptibility to stress corrosioncracking increases and formability declines. Further, this contributesto rising costs. Thus, it is preferable to include Ni in an amount of0.2 to 5%.

Mo: Mo may be included as necessary to improve resistance to crevicecorrosion. Mo is particularly effective against the generation ofcrevice corrosion. In addition to it, the effect on suppressing the rateof progression after crevice corrosion occurs is enhanced when Mo isadded in combination with Sn or Sb, or in combination with Ni. Thus, itis possible to improve resistance to penetration hole formation at acrevice portion (resistance to crevice corrosion). This effect becomesstable at an amount of 0.3% or more. This effect of Mo is enhanced asthe amount of Mo is increased, however, when Mo is included in excess,Mo causes a deterioration in formability and contributes to rising costsbecause Mo is expensive. Thus, it is preferable to include Mo in anamount of 0.3 to 3%.

Cu: Cu may be included as necessary to ensure resistance to crevicecorrosion. Cu is effective for decreasing the rate of progression aftercrevice corrosion occurs, and it is preferable to include Cu in anamount of 0.1% or more. However, when Cu is included in excess,formability deteriorates. Accordingly, it is preferable to include Cu inan amount of 0.1 to 1.5%.

V: V may be included as necessary for the purpose of further improvingresistance to crevice corrosion. Similar to Mo, V is effective againstthe generation of crevice corrosion and is also effective for decreasingthe rate of progression after crevice corrosion occurs. This effectbecomes stable at an amount of 0.02% or more. This effect is enhanced asthe amount of V is increased, however, when V is included in excess, Vleads to rising costs. Therefore, it is preferable to include V in anamount of 0.02 to 3.0%.

W: W may be included as necessary for the purpose of further improvingresistance to crevice corrosion. Similar to Mo and V, W is effectiveagainst the generation of crevice corrosion and is also effective fordecreasing the rate of progression after crevice corrosion occurs. Thiseffect becomes stable at an amount of 0.3% or more. This effect isenhanced as the amount of W is increased, however, when W is included inexcess, W leads to rising costs Therefore, it is preferable to include Win an amount of 0.3 to 5.0%.

Al: Al has deoxidizing effects and is a useful element in refining. Italso improves formability. Therefore, it is preferable to include Al inan amount of 0.003 to 1%.

Ca: As in the case of Al, Ca has deoxidizing effects and is a usefulelement in refining. It is preferable to include Ca in an amount of0.0002 to 0.002%.

Mg: As in the case of Al and Ca, Mg has deoxidizing effects and is auseful element in refining. It also refines the structure and iseffective in improving formability and toughness. Accordingly, it ispreferable to include Mg in an amount of 0.0002 to 0.002%.

B: B is an element useful for improving the secondary formability. It ispreferable to include B in an amount of 0.0002 to 0.005%.

EXAMPLES Example 1

Steels having the chemical compositions shown in Tables 1 and 2 weresmelted, and these steels were subjected to a process of hot-rolling,annealing of hot-rolled plates, cold-rolling, and finish annealing so asto produce steel plates having the thickness of 1.0 mm. Using thesecold-rolled steel plates, the corrosion resistance and the ductility atroom temperature were evaluated.

TABLE 1 Finish Chemical Composition of Test Steel (mass %) annealing No.C Si Mn P S Cr Ni Ti Nb N Other (° C.) A1 0.005 0.24 0.12 0.025 0.00120.12 3.04 0.19 0.011 0.006 — 1050 A2 0.006 0.22 0.20 0.028 0.001 20.343.02 0.004 0.25 0.007 — 1050 A3 0.007 0.14 0.15 0.026 0.002 19.66 3.110.17 0.008 0.009 1.23 Mo, 1025 0.023 Al, 0.0005 B A4 0.006 0.27 0.180.022 0.001 21.12 3.45 0.18 0.26 0.007 0.89 Mo 1000 A5 0.005 0.14 0.170.021 0.001 19.84 3.22 0.20 0.29 0.006 1.12 Mo, 1050 0.29 Nb, 0.41 V,0.0005 Mg 0.0004 B A6 0.004 0.22 0.16 0.022 0.001 22.44 4.12 0.19 0.0090.007 0.99 Mo, 1050 0.25 Cu A7 0.004 0.13 0.12 0.023 0.001 18.22 3.320.16 0.012 0.007 1.00 Mo, 1050 0.88 W, 0.32 Zr A8 0.015 0.08 0.35 0.0180.007 16.51 3.15 0.001 0.25 0.003 0.15 V, 1060 0.99 Al, 0.0034 B A90.003 0.42 0.06 0.038 0.006 24.01 4.87 0.41 0.001 0.018 2.1 Mo, 10100.34 W, 0.0011 Ca, 0.0018 Mg

TABLE 2 Finish Chemical Composition of Test Steel (mass %) annealing No.C Si Mn P S Cr Ni Ti Nb N Other (° C.) A10 0.017 0.12 0.13 0.018 0.00319.00 3.93 0.13 0.21  0.006 0.51 Cu 1030 2.21 W 0.10 Zr 0.34 Al 0.0037 BA11 0.011 0.23 0.07 0.031 0.005 12.30 3.05 0.35 0.22  0.014 0.51 Mo 10201.98 V 0.79 Al 0.0018 Ca 0.0002 Mg A12 0.004 0.11 0.13 0.024 0.001 18.313.01 0.19 0.001 0.006 1.09 Mo 980 0.46 Al 0.0004 E A13 0.011 0.35 0.470.002 0.008 23.15 4.44  0.002 0.45  0.013 0.20 V 1020 0.25 Al A14 0.0040.21 0.16 0.024 0.002 19.26 2.23 0.16 0.015 0.008 — 1000 A15 0.006 0.320.16 0.024 0.001 20.26 5.45 0.12 0.004 0.006 — 1000 A16 0.005 0.12 0.130.025 0.001 18.22 3.12 0.17 0.006 0.008 — 1150 A17 0.04  0.45 0.89 0.0240.004 18.12 8.22  0.005 0.007 0.04  (SUS304) 1050 A18 0.016 1.92 0.610.019 0.001 18.14 10.15   0.008 0.008 0.05  (SUS315J1) 1050 Note:Underline indicates a value that is outside the range of the presentinvention.

(Resistance to Crevice Corrosion)

A test piece having the width of 60 mm and the length of 130 mm and atest piece having the width of 30 mm and the length of 60 mm were cutfrom the cold-rolled steel. Wet polishing was then carried out usingemery paper #320. These large test piece and small test piece were thenstacked and were spot-welded at two points, such as shown in FIG. 1((positions (spot welding sites 1) indicated by O in FIG. 1). The endsurfaces and the rear surface of the test piece having the width of 60mm and the length of 130 mm were covered with sealing tape.

Using these test pieces, a repeated drying and wetting test was carriedout under the conditions indicated in FIG. 2. The spray solution was a5% calcium chloride aqueous solution. During the test cycle, aconcentrated calcium chloride environment was provided from the timewhen the process was switched from the spraying process to the dryingprocess until the inside of the crevice became completely dry. Inaddition, chloride ions were deposited inside the crevice as the cycleprogressed; thereby, this also provided a concentrated calcium chlorideenvironment. After the completion of 300 cycles, the large and smalltest pieces were separated. Next, corroded products were removed, anddepths of corrosion at the spot welded crevice portions were measuredusing the focal depth method. In addition to the conditions prescribedhere, testing was carried out in conformity with JASO M609-91 which isthe corrosion testing method for automobile materials prescribed bySociety of Automotive Engineers of Japan. The maximum value forcorrosion depth was obtained from among corrosion depth values measuredat 10 or more points. In the case in which the maximum value was 400 μmor less, the test piece was rated as “good”, and in the case in whichthe maximum value was more than 400 μm, the test piece was rated as“bad”. The thicknesses of the stainless steel plates employed in thesalt-induced corrosion environment which is the subject of the presentinvention are mainly in the range of 0.8 to 2 mm, and therefore, thethickness of 400 μm which is one half the thinnest thickness was takenas the standard.

(Resistance to Stress Corrosion Cracking)

Test pieces having the width of 15 mm and the length of 75 mm were cutout from the cold-rolled steel plate parallel to the rolled direction.The test pieces were bent at the curvature of 8R, and were bundled inparallel so as to form a U-bend test piece. 10 μl of artificial seawaterwas then dripped onto two sites on the outer surface of the R portion ofthe U-bend test piece. The U-bend test piece was placed in athermohydrostatic tester in a state where the R portion of the U-bendtest piece was directed upward, and was maintained for 672 hours at 80°C. and 40% RH. Under these conditions, the sodium chloride contained inthe artificial seawater was completely dried, to form a concentratedmagnesium chloride environment. After the test was completed, the outersurface and the cross-section of the R portion of the test piece wereobserved and evaluated whether stress corrosion cracking was present orabsent.

(Microstructure and Ductility at Room Temperature)

The ratio of the second phase including martensite phase and austenitephase was determined by image analysis based on pictures of thecross-sectional microstructure at 500-fold magnification. The grain sizenumber of ferrite phase was measured in accordance with JISG 0552.

Ductility at room temperature was measured by obtaining pieces for JIS13B tensile testing that were obtained parallel to the rolled directionfrom the test pieces described above. These test pieces were thensubjected to room temperature tensile testing; thereby, total elongationwas measured. A target of 20% was established for total elongation whichis desirable value for formation of components such as buildingmaterials, outside equipments, fuel tanks and pipes for automobiles andtwo-wheeled vehicles, and the like, that are the subjects of the presentinvention.

These test results are shown in Table 3.

TABLE 3 Resistance to Resistance to stress Ratio of second Grain sizeElongation at room No. crevice corrosion corrosion cracking phase (%)number temperature (%) A1 good good 0 7 27.8 A2 good good 0 7 28.2 A3good good 0   7.5 25.6 A4 good good 0 6 23.4 A5 good good 0 7 24.6 A6good good 12    6.5 21.5 A7 good good 0 7 24.2 A8 good good 1   7.5 23.5A9 good good 0   8.5 24.3 A10 good good 5 9 22.9 A11 good good 0 8 26.3A12 good good 0 8 29.8 A13 good good 0   7.5 25.3 A14 bad good 0 7 28.9A15 good good 50  9 12.5 A16 good good 0   3.5 18.5 A17 good bad 100  858.2 A18 good bad 100  7 54.2 (Note) Underline indicates cases where theratio of the second phase exceeded 15% or the grain size number was lessthan No. 4.

The steels of No. A1 to No. A13, which are within the scope of thepresent invention, had maximum corrosion depths of 400 μm or less at thecrevice portions. In addition, these steel samples did not experiencecracking during the test for stress corrosion cracking, and demonstratedexcellent corrosion resistance, as well as these steel samples hadelongations at room temperature of 20% or more, and had excellentformability.

The steel of No. A14, in which the Ni amount was out of the rangeprescribed for the present invention, had good resistance to stresscorrosion cracking and good elongation at room temperature, but hadinferior resistance to crevice cracking. The steel of No. A15, in whichthe Ni amount and the ratio of the second phase were out of the rangesprescribed for the present invention, had good resistance to crevicecorrosion and good resistance to stress corrosion cracking, but theelongation at room temperature was less than 20% and therefore, theformability was bad. The steel of No. A16, in which the grain sizenumber was less than No. 4, had the elongation at room temperature ofless than 20% and therefore, the formability was bad. The steels of Nos.A17 and A18 correspond to SUS 304 and SUS 315J1 steels, respectively.These steels had good resistance to crevice corrosion, but experiencedcracking during the tests for stress corrosion cracking and thus wereinferior in resistance to stress corrosion cracking.

Example 2

Steels having the chemical compositions shown in Table 4 were smelted,and these steels were subjected to a process of hot-rolling,cold-rolling and annealing so as to produce steel plates having thethickness of 1.0 mm. Using these cold-rolled steel plates, resistance tocrevice corrosion, formability, and resistance to ridging wereevaluated.

TABLE 4 Composition (mass %) No C Si Mn P S Ni Cr Mo Ti Nb Al N OtherInventive B1 0.001 0.12 0.09 0.028 0.0012 0.4 20.8 1.0 0.14 0.014 0.250.010 Example B2 0.004 0.35 0.21 0.024 0.0004 0.6 17.4 1.5 0.15 0.0030.34 0.009 0.06 V, 0.0003 B B3 0.013 0.78 0.14 0.034 0.0021 1.0 19.2 1.20.35 0.002 0.68 0.010 0.0002 Mg, 0.0006 B B4 0.004 0.05 0.19 0.0150.0055 2.0 17.9 0.6 0.19 0.002 0.89 0.010 0.0002 Ca B5 0.002 0.12 0.350.015 0.0003 0.3 16.5 2.1 0.17 0.005 0.22 0.013 0.12 V B6 0.004 0.100.11 0.028 0.0011 2.9 18.1 1.0 0.21 0.001 0.12 0.008 0.0005 B B7 0.0180.11 0.88 0.033 0.0079 0.4 18.0 1.0 0.42 0.003 0.16 0.011 0.15 Cu,0.0011 Ca, 0.0011 B B8 0.011 0.39 0.68 0.038 0.0014 2.0 19.9 0.5 0.210.033 0.42 0.009 0.23 Cu, 2.10 V B9 0.005 0.10 0.12 0.011 0.0025 3.018.1 0.7 0.25 0.045 0.68 0.016 0.0041 Ca B10 0.003 0.23 0.15 0.0260.0011 2.9 14.5 1.8 0.32 0.004 0.11 0.007 B11 0.009 0.11 0.77 0.0380.0022 2.5 21.1 2.6 0.18 0.071 0.89 0.004 0.0039 Mg, 0.0048 B B12 0.0010.05 0.06 0.019 0.0033 2.2 20.4 0.6 0.25 0.022 0.31 0.008 1.35 Cu B130.002 0.39 0.24 0.025 0.0005 2.8 16.3 0.8 0.07 0.002 0.9  0.004 0.51 VComparative B14 0.004 0.11 0.10 0.027 0.0007  0.03 17.9 1.0 0.13 0.0140.21 0.011 0.0034 Ca, 0.0028 Mg Example B15 0.002 0.53 0.09 0.035 0.00090.2 17.5 0.3 0.35 0.002 0.15 0.016 0.32 Cu, 0.0003 Mg B16 0.001 0.250.65 0.021 0.0012 1.2 16.5 2.1 0.21 0.055 0.06 0.009 0.005 B B17 0.0090.05 0.25 0.019 0.0055 2.1 19.5 1.8 0.29 0.12  0.25 0.016 0.2 V Note:Underline indicates a value that is outside the range of the presentinvention

(Resistance to Crevice Corrosion)

A test piece having the width of 60 mm and the length of 130 mm and atest piece having the width of 30 mm and the length of 60 mm were cutfrom the cold-rolled steel. Wet polishing was then carried out usingemery paper #320. The test pieces were spot-welded into the form shownin FIG. 1, and the end surfaces and the rear surface of the test piecehaving the width of 60 mm and the length of 130 mm were covered withsealing tape. Using these test pieces, a repeated drying and wettingtest was carried out under the conditions indicated in FIG. 3. After thecompletion of 180 cycles, the large and small test pieces wereseparated. Next, the corroded products were removed, and depth ofcorrosions at the spot welded crevice portions were measured using anoptical microscope focal depth method. In addition to the conditionsprescribed here, testing was carried out in conformity with JASO M609-91which is the corrosion testing method for automobile materialsprescribed by Society of Automotive Engineers of Japan.

The maximum value for corrosion depth was obtained from among corrosiondepth values measured at 10 or more points. In the case in which themaximum value was 800 μm or less, the test piece was rated as “good”,and in the case in which the maximum value was more than 800 μm, thetest piece was rated as “bad”. The thicknesses of the stainless steelplates which are the subject of the present invention are mainly in therange of 0.8 to 2.0 mm, and therefore, the thinnest thickness was takenas the standard.

(Formability)

Formability was evaluated by a cylindrical deep drawing test. Theforming conditions were as follows. Punch diameter: φ50 mm; punchshoulder R: 5 mm; dice shoulder R: 5 mm; blank diameter: φ100 mm; blankholder force: 1 ton; and friction coefficient: 0.11 to 0.13. Here, thisfriction coefficient is the level obtained by coating lubricating oil tothe front and the rear surface of the steel sheet at a kinematicviscosity of 1200 mm²/mm at 40° C. Formability was evaluated based onwhether or not it was possible to carry out deep drawing formation at aforming limit drawing ratio of 2.20 under the conditions describedabove. In other words, in the case in which formation was possible, thesteel was rated as “good”. In the case in which formation cracksoccurred during the process, the steel was rated as “bad”.

(Resistance to Ridging)

Resistance to ridging was evaluated using tensile test pieces obtainedfrom the cold-rolled steel plate parallel to the rolled direction. Thesetest pieces were elongated by 15%, and then surface irregularities(waviness) in the rolled direction and in the vertical direction weremeasured using a two-dimensional roughness meter. The maximum height ofthe irregularities was defined as the ridging height. In the case inwhich the ridging height was less than 15 μm, the steel was rated as“good”. In the case in which the ridging height was 15 μm or more, thesteel was rated as “bad”.

These test results are shown in Table 5.

TABLE 5 Value of Value of Resistance to Resistance to No. Formula (A)Formula (B) crevice corrosion Formability ridging Comments B1 26.2 18good good good Inventive Example B2 25.9 113  good good good InventiveExample B3 28.8 340  good good good Inventive Example B4 31.7 445  goodgood good Inventive Example B5 25.3 44 good good good Inventive ExampleB6 38.5 120  good good good Inventive Example B7 24.3 53 good good goodInventive Example B8 47.4 13 good good good Inventive Example B9 38.2 15good good good Inventive Example B10 37.3 28 good good good InventiveExample B11 43.9 13 good good good Inventive Example B12 43.5 14 goodgood good Inventive Example B13 38.6 450  good good good InventiveExample B14 21.1 15 bad good good Comparative Example B15 21.5 75 badgood good Comparative Example B16 30    1 good bad bad ComparativeExample B17 38.7  2 good bad bad Comparative Example Note: Underlineindicates a value outside the range of the present invention.

The steels of No. B1 to No. B13, which are within the scope of thepresent invention, had excellent resistance to crevice corrosion,excellent formability, and excellent resistance to ridging.

The steel of No. B14, in which the Ni amount and the value of Formula(A) were out of the ranges prescribed for the present invention, and thesteel of No. B15, in which the Mo amount and the value of Formula (A)were out of the ranges prescribed for the present invention, hadinferior resistance to crevice corrosion. Further, the steel of No. B16,in which the Al amount and the value of Formula (B) were out of theranges prescribed for the present invention, had inferior resistance toridging. The steel of No. B17, in which the Nb amount and the value ofFormula (B) were out of the ranges prescribed for the present invention,had both of inferior formability and inferior resistance to ridging.

From the above examples, the effects of the present invention were thusconfirmed.

Example 3

Steels having the chemical compositions shown in Table 6 were smelted,and these steels were subjected a process of to hot-rolling,cold-rolling and annealing so as to form steel plates having thethickness of 1.0 mm. Using these cold-rolled steel plates, resistance tocrevice corrosion were evaluated.

TABLE 6 Composition (mass %) No. C Si Mn P S Ni Cr Ti Nb Sn Sb InventiveC1 0.005 0.38 0.26 0.027 0.001 16.21 0.25 0.41 Example C2 0.008 0.360.25 0.025 0.001 15.99 0.23 0.22 C3 0.005 0.35 0.35 0.026 0.002 0.2116.62 0.18 0.35 C4 0.012 0.12 0.25 0.020 0.001 17.28 0.25 0.28 C5 0.0030.49 0.65 0.016 0.005 0.36 18.25 0.20 0.49 C6 0.008 0.25 0.12 0.0320.002 0.68 13.56 0.18 0.25 0.03 C7 0.005 0.18 0.16 0.025 0.001 1.0018.20 0.19 0.22 0.13 C8 0.007 0.26 0.36 0.029 0.001 1.26 19.46 0.20 0.007 C9 0.003 0.21 0.32 0.021 0.001 1.46 17.69 0.16 0.20  0.006 C100.006 0.16 0.22 0.024 0.001 1.76 19.68 0.36 0.01  0.006 C11 0.004 0.130.22 0.023 0.008 2.03 20.25 0.32 0.04 C12 0.006 0.08 0.10 0.022 0.0014.60 24.56 0.22 0.01 C13 0.005 0.42 0.75 0.028 0.001 0.25 15.22 0.120.26 0.76 Comparative C14 0.004 0.42 0.22 0.025 0.004 14.86 0.26  0.003Example C15 0.007 0.12 0.16 0.021 0.002 15.22 0.35  0.002 C16 0.006 0.420.36 0.028 0.003 10.95 0.20 0.33 Composition (mass %) No. N Mo Cu V W AlCa Mg B Inventive C1 0.011 Example C2 0.009 C3 0.008 C4 0.015 1.15 0.030.0005 C5 0.004 0.44 0.01 0.0005 C6 0.011 0.78 2.50 0.15 0.0010 C7 0.0080.99 0.06 0.0003 C8 0.009 1.05 0.01 0.0006 0.0004 C9 0.008 1.43 0.220.0005 0.0005 C10 0.012 0.82 0.04 0.0006 C11 0.006 0.46 0.0004 C12 0.0052.66 C13 0.016 1.23 0.35 0.0004 Comparative C14 0.008 0.05 Example C150.009 C16 0.008 Note: Underline indicates a value outside the range ofthe present invention.

A test piece having the width of 60 mm and the length of 130 mm and atest piece having the width of 30 mm and the length of 60 mm were cutfrom the cold-rolled steel. Wet polishing was then carried out usingemery paper #320. The test pieces were spot-welded into the form shownin FIG. 1, and the end surfaces and the rear surface of the test piecehaving the width of 60 mm and the length of 130 mm were covered withsealing tape.

Using these test pieces, a repeated drying and wetting test was carriedout under the conditions indicated in FIG. 7. After the completion of120 cycles, the large and small test pieces were separated. Next, thecorroded products were removed, and depth of corrosions at the spotwelded crevice portions were measured using an optical microscope focaldepth method. The maximum value was obtained from among corrosion depthvalues measured at 10 or more points where deep corrosion appeared tohave occurred. In addition to the conditions prescribed here, testingwas carried out in conformity with JASO M609-91 which is the corrosiontesting method for automobile materials prescribed by Society ofAutomotive Engineers of Japan.

These test results are shown in Table 7.

TABLE 7 Maximum corrosion No. depth (μm) Inventive C1 516 Example C2 534C3 487 C4 402 C5 376 C6 397 C7 213 C8 205 C9 188 C10 168 C11 336 C12 138C13 356 Comparative C14 846 Example C15 875 C16 925

The steels of No. C1 to No. C13, which are within the scope of thepresent invention, had maximum corrosion depths of 600 μm or less, andtherefore, their resistances to crevice corrosion were excellent. Thesteel of No. C14 in which the Sn amount was out of the range prescribedfor the present invention, the steel of No. C15 in which the Sb amountwas out of the range prescribed for the present invention, and the steelof No. C16 in which the Cr amount was out of the range prescribed forthe present invention, had maximum corrosion depths of 800 μm or more,and therefore, their resistances to crevice corrosion were inferior.From the above examples, the effects of the present invention were thusconfirmed.

INDUSTRIAL APPLICABILITY

The first embodiment of the present invention is suitable for buildingmaterials and outside equipments in a marine environment where airbornesalt is ubiquitous, as well as for component parts of automobiles andtwo-wheeled vehicles which travel over cold regions where antifreezingagents are spread in winter.

The ferritic stainless steel having excellent resistance to penetrationhole formation at crevice portions (resistance to crevice corrosion) andsuperior formability according to the second embodiment of the presentinvention is useful for components where crevices are present in thedesign, and where superior resistance to crevice corrosion and superiorformability are required, such as exhaust system components and fuelsystem components of automobiles and two-wheeled vehicles, hot-watersupply equipments, and the like. In particular, this ferritic stainlesssteel is suitable for important components where a long lifespan isrequired, such as automobile fuel tanks and fuel oil supply pipes.

The ferritic stainless steel having excellent resistance to crevicecorrosion, and particularly excellent resistance to penetration holeformation at crevice portions, according to the third embodiment of thepresent invention, is useful as a material employed in components thatrequire superior resistance to crevice corrosion, in equipments andpipings that have crevice portions in their design and are used inchloride environments, such as automobile components, water and hotwater supply equipments, building equipments, and the like.

1. A stainless steel excellent in corrosion resistance, comprising, interms of mass %, C: 0.001 to 0.02%, N: 0.001 to 0.02%, Si: 0.01 to 0.5%,Mn: 0.05 to 0.5%, P: 0.04% or less, S: 0.01% or less, Ni: more than 3%to 5%, and Cr: 11 to 26%, and further comprising either one or both ofTi: 0.01 to 0.5% and Nb: 0.02 to 0.6%, and containing as the remainder,Fe and unavoidable impurities.
 2. A stainless steel excellent incorrosion resistance according to claim 1, which further comprises oneor more selected from the group consisting of Mo, Cu, V, W, and Zr,within the amounts of Mo: 3.0% or less, Cu: 1.0% or less, V: 3.0% orless, W 5.0% or less, and Zr: 0.5% or less.
 3. A stainless steelexcellent in corrosion resistance according to claim 1, which furthercomprises one or more selected from the group consisting of Al: 1% orless, Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% or less. 4.A stainless steel excellent in corrosion resistance according to claim3, wherein the combined ratio of austenite phase and martensite phase is15% or less, ferrite phase is included as the remainder, and the grainsize number of the ferrite phase is No. 4 or greater.
 5. A stainlesssteel excellent in corrosion resistance according to claim 1, whereinthe combined ratio of austenite phase and martensite phase is 15% orless, ferrite phase is included as the remainder, and the grain sizenumber of the ferrite phase is No. 4 or greater.
 6. A ferritic stainlesssteel excellent in resistance to crevice corrosion and formability,comprising, in terms of mass %, C: 0.001 to 0.02%, N: 0.001 to 0.02%,Si: 0.01 to 1%, Mn: 0.05 to 1%, P: 0.04% or less, S: 0.01% or less, Ni:0.15 to 3%, Cr: 11 to 22%, Mo: 0.5 to 3%, Ti: 0.01 to 0.5%, Nb: lessthan 0.08%, and Al: more than 0.1% to 1%, and containing as theremainder, Fe and unavoidable impurities, wherein the amounts of Cr, Ni,Mo and Al satisfy the following formulas (A) and (B).Cr+3Mo+6Ni≧23  (A)Al/Nb≧10  (B)
 7. A ferritic stainless steel excellent in resistance tocrevice corrosion and formability according to claim 6, which furthercomprises either one or both of Cu: 0.1 to 1.5% and V: 0.02 to 3.0% atthe amounts which satisfy the following formula (A′).Cr+3Mo+6(Ni+Cu+V)≧23  (A′)
 8. A ferritic stainless steel excellent inresistance to crevice corrosion and formability according to claim 6,which further comprises one or more selected from the group consistingof Ca: 0.0002 to 0.002%, Mg: 0.0002 to 0.002%, and B: 0.002 to 0.005%.9. A ferritic stainless steel excellent in resistance to crevicecorrosion, comprising, in terms of mass %, C: 0.001 to 0.02%, N: 0.001to 0.02%, Si: 0.01 to 0.5%, Mn: 0.05 to 1%, P: 0.04% or less, S: 0.01%or less, and Cr: 12 to 25%, further comprising either one or both of Tiand Nb within the amounts of Ti: 0.02 to 0.5% and Nb: 0.02 to 1%,further comprising either one or both of Sn and Sb within the amounts ofSn: 0.005 to 2% and Sb: 0.005 to 1%, and containing as the remainder, Feand undetectable impurities.
 10. A ferritic stainless steel excellent inresistance to crevice corrosion according to claim 9, which furthercomprises either one or both of Ni: 5% or less and Mo: 3% or less.
 11. Aferritic stainless steel excellent in resistance to crevice corrosionaccording to claim 9, which further comprises one or more selected fromthe group consisting of Cu: 1.5% or less, V: 3% or less, and W: 5% orless.
 12. A ferritic stainless steel excellent in resistance to crevicecorrosion according to claim 11, which further comprises one or moreselected from the group consisting of Al: 1% or less, Ca: 0.002% orless, Mg: 0.002% or less, and B: 0.005% or less.
 13. A ferriticstainless steel excellent in resistance to crevice corrosion accordingto claim 9, which further comprises one or more selected from the groupconsisting of Al: 1% or less, Ca: 0.002% or less, Mg: 0.002% or less,and B: 0.005% or less.